A typical cellular wireless network includes a number of base stations each radiating to define a respective coverage area in which user equipment devices (UEs) such as cell phones, tablet computers, machine-type-communication devices, tracking devices, embedded wireless modules, and/or other wirelessly equipped communication devices (whether or not user operated), can operate. In turn, each base station could be coupled with network infrastructure that provides connectivity with one or more transport networks, such as the public switched telephone network (PSTN) and/or the Internet for instance. With this arrangement, a UE within coverage of the network could engage in air interface communication with a base station and could thereby communicate via the base station with various remote network entities or with other UEs served by the base station.
Further, a cellular wireless network may operate in accordance with a particular air interface protocol or radio access technology, with communications from the base stations to mobile terminals defining a downlink or forward link and communications from the UEs to the base stations defining an uplink or reverse link. Examples of existing air interface protocols include 3G technologies such as Code Division Multiple Access (CDMA) and Global System for Mobile Communication (GSM), 4G technologies such as Long Term Evolution (LTE) (using orthogonal frequency division multiple access (OFDMA) on the downlink and single-carrier frequency division multiple access (SC-FDMA) on the uplink, and emerging 5G technologies (such as 5G NR (5G New Radio)), among others. Each protocol may define its own procedures for registration of UEs, initiation of communications, handover between coverage areas, and other functions related to air interface communication.
In accordance with the air interface protocol, a base station may provide service on one or more carrier frequencies, referred to as carriers. Each such carrier could be frequency division duplex (FDD), with separate frequency channels for downlink and uplink communication, or time division duplex (TDD), with a single frequency channel multiplexed over time between downlink and uplink use. Representative frequency channels could each have a bandwidth (width of frequency spectrum), such as 1.25 MHz, 3 MHz, 5 MHz, 10 MHz, 15 MHz, or 20 MHz, among other possibilities.
On each such carrier, the downlink and uplink could each be structured to define air interface resources and sub-channels for carrying information between the base station and UEs. For instance, on the downlink, certain resources could be reserved to carry a reference signal that UEs can measure to evaluate coverage quality, other resources could be reserved to carry other control signaling to UEs, and still other resources could be reserved to carry bearer traffic (e.g., application-layer communications) from the base station to UEs. Likewise, on the uplink, certain resources could be reserved to carry control signaling such as access requests and scheduling requests from UEs to the base station, and other resources could be reserved to carry bearer traffic from UEs to the base station.
When a UE first powers on or otherwise enters into coverage of such a system, the UE could scan for a best coverage area in which to operate and could then engage in signaling to acquire wireless connectivity with the base station that provides that coverage area. For instance the UE could scan various carriers in an effort to find the carrier having the strongest or best quality reference signal as measured by the UE. And the UE could then engage in signaling to establish a radio-link-layer connection with the associated base station on the detected carrier and to register for service with the network via that base station.
Once the UE is so connected and registered, the base station could then provide the UE with data communication service (including possibly voice-over-packet service and other packet-based media services). For instance, when the core network has data destined to the UE, the base station could schedule transmission of that data on a downlink traffic channel of the UE's serving carrier and could accordingly transmit the data to the UE. And when the UE has data for transmission to the core network, the base station could schedule transmission of that data on an uplink traffic channel of the UE's serving carrier, and the UE could accordingly transmit the data to the base station
Optimally, a wireless service provider will strategically implement base stations throughout a market area so that served UEs can transition between the base stations' coverage areas without experiencing a loss of coverage. Each base station may include an antenna structure and associated equipment, and the service provider may connect each base station by a landline cable (e.g., a T1 line) with the service provider's core network, to enable the base station to communicate on that network.
In some situations, however, it may be impractical for a wireless service provider to run landline connections to base stations. For instance, where a service provider seeks to provide many small coverage areas blanketing a market area or to fill in coverage holes between coverage of other base stations, the service provider may implement many small-cell base stations throughout the market area, but it may be inefficient or undesirable to run landline cables to every one of those small-cell base stations.
To provide coverage in such locations, the wireless service provider may instead implement relays, each of which could be configured to operate in much the same way as a conventional landline-connected base station but could have a wireless backhaul connection to a core network. In particular, each relay could include a relay base station and a UE module referred to as a “UE-relay” (integrated or communicatively linked together). The UE-relay, and thus the relay, could then be served by an existing base station of the network, referred to in that scenario as a donor base station, with the air interface between the UE-relay and the donor base station defining a wireless backhaul connection for the relay. With this arrangement, the relay could thus conveniently communicate with the core network (e.g., with other entities on the core network) via the wireless backhaul connection and the donor base station.
As with conventional UEs (e.g., end-user UEs), when a UE-relay first powers on or otherwise enters into coverage of the network, the UE-relay could discover coverage provided by a base station on a given carrier, and the UE-relay could connect with the base station on that carrier and register for service with the network. The base station, functioning as a donor base station, could then serve the UE-relay as discussed above, and the relay base station could serve one or more end-user UEs. Thus, data from the core network to an end-user UE would flow over the UE-relay's serving carrier from the donor base station to the UE-relay and would then flow from the relay base station to the end-user UE. And data from the end-user UE to the core network would flow from the end-user UE to the relay base station and would then flow over the UE-relay's serving carrier to the donor base station.